We are interested in making FDPM measurements to probe layered tissue structures as well as assess the properties of small tissue volumes. The specific aims are 1) Generalize standard optical diffusion theory (SODT) to provide governing equations which accommodate spatially distributed collimated sources, 2) Solve these new equations for steady and amplitude-modulated collimated point sources located within an infinite medium. Compare results to solutions derived using SODT and experimental measurement, 3) Solve the new equations for steady and amplitude-modulated collimated sources illuminating the surface of an infinite medium. Compare results with SODT and experiment, and 4) Develop a theoretical framework to address cases where FDPM measurements contain significant contributions from both minimally scattered and fully diffuse photons.